1. Field of the Invention
The present invention relates to a novel polyethylene synthetic pulp. More particularly, the invention relates to such a polyethylene synthetic pulp that gives synthetic paper which may be, after being made in a papermaking machine, heat-treated with a heating device attached to a conventional papermaking machine.
In the specification, by "polyethylene" it is meant not only homopolymers of etheylene, but also copolymers of ethylene with a minor amount of other olefins such as propylene, 1-butene and 4-methyl-1-pentene.
2. State of the Art
Synthetic pulp comprising short fibers of polyethylene is made into paper, as is, or usually, as a mixture with wood pulp or other materials, to form synthetic paper which is applied for various uses. The uses are found in various fields of industry, such as papermaking, e.g., water-resistant corrugated cardboard, water-resistant white cardboard, embossed paper, heat-sealing paper and battery separators; dry binders and dry non-woven cloth, e.g., felt mats and hygienic absorbents; and building materials mainly for replacement of conventional asbestos products.
To meet the demands in variety, many grades of synthetic pulp are prepared. Synthetic pulp products use polyethylenes of different physical properties, such as specific gravity, softening point or melting point, heat of fusion and mechanical strength, and properties depending on the shape of the pulp fibers, such as fiber length, diameter, surface area and filtrage.
One of the merits of synthetic pulp is that the material polyethylene is thermoplastic, and therefore, that the product synthetic paper obtains higher strength and smoothness, lower moisture permeability, and further, that the synthetic paper becomes transparent and may be processed by deep-drawing.
In the production of synthetic paper using a polyethylene synthetic pulp now on the market, the synthetic pulp is usually mixed in an appropriate ratio with wood pulp, and the mixture is made into paper in a conventional papermaking machine which is used for making plain paper. It is necessary to heat-treat the synthetic paper thus made, after drying, in a special heating device so that the synthetic paper may exhibit the above-noted properties.
Dryers and calendering rolls in conventional papermaking machines are heated by steam and operated at a drying temperature ranging from 90.degree. C. to 100.degree. C., usually a little higher than 95.degree. C.
Grades of conventional polyethylene synthetic pulp are divided broadly into two groups from the viewpoint of the material: those with high-density polyethylene and those with medium-density polyethylene. None of these polyethylenes melt or soften at a temperature in the above-noted range of operating temperatures of conventional heating devices. Thus, heat treatment of the synthetic pulp requires a special device.
In order to produce synthetic pulp suitable for papermaking, it was proposed to flash-spin a mixture of homogeneous phases of two different polymers (Japanese Patent Publication No. 32123/1982 or U.S. Pat. No. 4,260,565). The proposed method is directed to formation of fibers with a higher extent of fibrillation, and the disclosure contains no solution for the above-noted problem in papermaking using the synthetic pulp.